A high resolution is required from applications in the low-voltage range, i.e. in the energy range of &lt;5 keV, preferably in the range of 500 eV to 2.5 keV, in particular with regard to the generation of a fine probe. An objective lens is known from EP-B-0 333 018, which can be used in the low-voltage range. The detector is disposed above the lens in this objective lens. The primary charged particles of a high first energy are decelerated to a lower final energy within this objective lens, e.g. down to 500 eV to 2.5 keV. The decelaration field necessary for this is generated by an electrostatic lens disposed within a magnetic lens.
It is true that the electric field strength can be adjusted in the area of the specimen by means of a control electrode disposed between the objective lens and the specimen, however, there is no further possibility of having an influence on the released charged particles. The magnetic field and the electrostatic field of the objective lens are only optimised with respect to the primary charged particle beam in order to obtain good resolution properties. Consequently, the influence of the released charged particles on the way to the detector is automatically determined. However, these conditions are sufficient for many applications.